NavList:

   

Reply

With respect to Mercator's polar map, and his proposed magnetic mountains,
Peter Fogg offered us a translation from a Russian website on the subject,
at a (transliterated) web address of-.
http://feng-shui.ua/articles/compass-declination.html

a machine-translation can be found at-


http://translate.google.com.au/translate?hl=en&langpair=ru|en&u=http://feng-shui.ua/articles/compass-declination.html

the heading of the article being "Where indicates the compass?"

The author was Alexander Mova, a name I have not come across before,
described as a "feng shui consultant", and the website in which it occurs
is one devoted to "feng shui". I understand that feng shui is some form of
Chinese necromancy, and one wouldn't expect much from an article about an
aspect of the history of science, found in such an environment. However,
his piece is rather well-informed, especially when allowances are made for
the deficiencies in machine translation.

It mentions the work of ART Jonkers, and the geomagnetic team at Leeds, and
my guess is that much of it derives from Jonkers' publications, though some
errors and misunderstandings occur.

======================================

John Huth has pointed us to a detailed Polar picture at-

http://en.wikipedia.org/wiki/File:Mercator_Septentrionalium_Terrarum_descriptio.jpg

This coloured illustration was produced in 1623 , an expanded version from
a corner-insert in Mercator's World map for navigators of 54 years before,
to which Barents' discoveries, around Novaya Zemlya (which occurred after
Mercator's death) had been added.

Not that it's relevant to our present enquiry, but I note that this 1623
map has its own expanded islands, in three of its corners. I can recognise
two, as the Faroes and Shetland, but what about the third, labelled
"Frisland"? Can anyone locate that? Doesn't look much like the Frisian
Islands, to me. Spitzbergen?

===========================

Woflgang gave us some really useful guidance, when he wrote-

"The map with the channels to the North Pole and the island marking one
magnetic pole first was an inset in the lower left corner of the famous
1569 Mercator map "ad usum navigantium". But it doesn't show one magnetic
pole but two: There are 2 smalls circles as you can clearly on the picture
provided by John: one with the inscription "Polus magnetis respectu Corvi
insule", the other is located on a small island with a mountain. Its
inscription reads: "Polus magnetis respectu insularum capitis Viridis". The
first inscription must be translated as: "Magnetic pole with respect to the
island of Corvo", the second as: "Magnetic pole with respect to the Cape
Verde islands".

The explanation for this is given by Mercator in the 1569 map also: In the
upper right corner of the map there is an inset where Mercator tries to
explain where he starts counting the longitude. Its title reads: "De
longitudinum geographicarum initio et polo magnetis" ("Of the beginning of
geographic longitudes and the magnetic pole"). Here he refers to the fact
that supposedly the line of zero variation passes through the Cape Verde
islands, but according to other authors through the Azores/Corvo. And he
states that the longitudes must be counted from the line of zero variation
i.e. either from Corvo or the Cape Verde islands. And therefore - depending
on which place you chose to start counting the longitude - the magnetic
pole is located in different places: as shown on his arctic maplet."

===================

But I would argue a bit about that. A change in longitude coordinates,
shifting the zero-point in longitude, should not in itself shift the
position of his additional pole with respect to its geographical
surroundings: it would only change its coordinates. The discrepancy was in
deciding at which longitude, in the Atlantic, the zero of magnetic
variation is to occur. Then, the second magnetic mountain (in addition to
the one at the North Pole) would be placed 180 degrees (in longitude) away
from it.

So I would take Mercator's meaning to be this-

If the zero-variation line was taken to pass through the Cape Verdes, then
it would call for the second magnetic mountain to be placed accordingly,
180 degrees away, and also for the Mercator's zero of longitude to pass
through those islands.

And if the zero-variation line was taken to pass through Corvo, in the
Azores, then the second magnetic mountain would be placed somewhat
differently in longitude to correspond, and the zero of longitude also to
be shifted, to pass through Corvo.

Really, all I'm saying here is that the choice of position of the second
mountain reflects the uncertainty in the magnetic observations, and the
change in longitude zero is a consequence of that magnetic uncertainty; not
in itself the reason why the mountain should be repositioned.

But I must admit to failing to understand why Mercator also shifts the
latitude of his mountain.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

Reply